Dynamic loudspeakers generally comprise a voice coil element that is positioned in an air gap in a magnetic field generated by one or more permanent magnets. The voice coil includes a current carrying conductor which, upon interacting with the magnetic field, is caused to move at right angles to the direction of the field. The voice coil is connected to a loudspeaker diaphragm such that the mechanical motion of the voice coil is translated to the diaphragm. By varying the current through the voice coil, it is possible to make the voice coil oscillate at different frequencies and so cause the diaphragm to produce sound of different frequencies.
For the loudspeaker to operate efficiently, the magnetic field in the air gap should be as strong as possible. Flux which emanates from the permanent magnets, but which does not contribute to the field experienced by the voice coil is considered as leakage.
In order to maintain a high efficiency, it is necessary to keep flux leakage to a minimum. Typically, therefore, loudspeakers employ ferromagnetic guide members to guide and/or focus the magnetic flux lines that emanate from the permanent magnets towards the air gap.
In some systems, additional permanent magnets are used for the ferromagnetic guide members. Other systems rely on ferromagnetic materials that are not permanent magnets to serve as the guide members.
In general, a guide member that is a permanent magnet can be distinguished from one which is not a permanent magnet by considering the guide member's ability to retain its magnetic properties after a magnetizing field has been removed. A permanent magnet will continue to retain its magnetic energy potential indefinitely after the field is removed. Once magnetised, it is difficult to alter the magnetic energy configuration of the material, or indeed, to demagnetise it altogether.
In addition to permanent magnets, there also exist other ferromagnetic materials, which retain only a small part of their magnetism once the magnetizing force is removed, and whose magnetic energy configuration is easily altered upon subsequent exposure to other magnetic fields. Where a ferromagnetic material ceases to retain its magnetic potential after such a field is removed, that piece of material is not a permanent magnet.
The difference between a permanent magnet and these other types of ferromagnetic material can also be discussed in terms of reluctance. The reluctance of a material defines the opposition that the material offers to magnetic lines of force, as those lines try to distribute themselves throughout the material. Once magnetised, a permanent magnet will have a high reluctance, in the sense that it will oppose any lines of force that are not aligned with its own intrinsic magnetic field. The converse to reluctance is permeability; permeability defines the ease with which magnetic lines of force distribute themselves throughout their material. Therefore, once magnetised, a permanent magnet will display low permeability to magnetic field lines which are not aligned with its own intrinsic magnetic field, whereas other ferromagnetic materials which display low reluctance will have a higher permeability.
An example of a system that uses additional permanent magnets to construct a closed magnetic loop between the two sides of the air gap is provided by US2009/0028375 A1. This document proposes a structure in which several permanent magnets having different placements and polarization orientations are used to channel the lines of magnetic flux in a loop that crosses the air gap.
Such systems suffer from several disadvantages. First, the permanent magnets are typically made from rare earth materials, which are both heavy and expensive. The use of additional permanent magnets to guide the magnetic flux therefore has both cost and weight implications for the magnetic structure.
Secondly, in the systems described above, the magnetic flux lines must, at several points in the loop, cross an interface between two permanent magnets that are magnetised in different directions to one another. As discussed above, the permanent magnets offer high reluctance to lines of flux that are not aligned with their intrinsic magnetic field. Thus, at the interface between two permanent magnets, the magnetic field lines are forced to undergo a sudden change in direction in order to progress to the next part of the loop. Where such sudden changes occur, it is inevitable that at least some of the magnetic flux will be lost to leakage.
As an alternative to using permanent magnets, other pieces of ferromagnetic material which are not themselves permanent magnets may be used to guide the flux. FIG. 1 shows a closed magnetic circuit in which a steel U-Yoke 1 is used to guide magnetic flux from a magnet 3 to an air gap 5 where a voice coil element 7 is placed, and back to the magnet again.
Steel parts such as the U-yoke described above are not permanent magnets; they do not contribute to the magnetic field strength of the magnet structure. They are, one might say, passive components in a magnet system, in which the permanent magnet is the source of energy. Rather than having their own intrinsic direction of magnetisation, the steel parts can adjust their magnetic configuration to accommodate lines of magnetic flux emanating from other parts of the system (in this case, the permanent magnet 3).
FIG. 2 shows a magnetic circuit representation of the circuit shown in FIG. 1 (note this analogy doesn't take saturation of the ferromagnetic materials into calculation). In FIG. 2, Fm is the magnetomotive force, Rm is the internal resistance (reluctance) and the air gap is represented by Rg. Since the steel yoke has a higher magnetic permeability than the surrounding air, most of the magnetic flux emanating from the permanent magnet(s) is channelled through the yoke rather than leaking out of the system and away from the air gap. In effect, the steel yoke serves as a low resistance conduit for channelling lines of magnetic flux from one part of the system to another.
By providing a guide member of the type shown in FIG. 2, the lines of magnetic flux can be made to circulate in a loop without having to negotiate the sudden changes in the magnetic environment that are present at the interface between permanent magnets. Nonetheless, if the geometry of the permanent magnets is poorly matched with that of the guide members, it is possible that magnetic flux will not be coupled effectively into the guide members, and will instead leak out of the system, or shortcut back to the permanent magnets. Where such losses occur, they reduce the overall field strength experienced by the voice coil, causing the performance of the loudspeaker to deteriorate.
In general, there is a continuing need to develop magnetic motor systems that provide enhanced magnetic field strength in the region of the voice coil.